Let me explain this picture a bit.
It's fairly hard to shove one tube inside another tube. It's especially hard as the lengths required get long and longer. When making a heat exchanger using 3/16 and 3/8" tube, it becomes an exercise in insanity.

Both tubes must be nearly perfectly straight.
That means, even the tinyiest imperfections become a problem. As you begin to push one through, friction becomes a huge problem. Even a degree or two at a tiny spot pushes parts of the tube against the inside of the other tube.
This isn't a problem with performance, simply that it becomes impossible to push one tube in any farther. You must then withdraw it, figure out what's stopping you, touch it a tiny little bit, and try again.

My method involves 5 ft long steel beams and clamps.

Clamp the tubes straight as possible.

And try.

When your looking down a 20ft long barrel though, things get interesting.
This took 3 days.

I think I went a bit overboard height wise. Don't need that much! (Yay the cascade just got shorter!) I think I'll shave 2 inches off of her. Having the "low" HX in the center helps with the insulation of it. Normally -80 to -90C would require 3" of foam.

Need to get cracking on getting that condenser for the first stage, I have a back up, but it's not as pretty or dense. Then I can worry the evaporator.

5 of 6 gauge kits.
2 ft of 1/8" tube on each, after brazing in an inch and a half of 1/4" tube to a tube adapator.
Will find that last bit or just pickup another while I'm at homedepot getting JB Weld for these joints. I don't like teflon tape, it's just not robust enough for me.